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The Internet-of-Things (IoT) landscape is expanding with new radio technologies. In addition to the Low-Rate Wireless Personal Area Network (LR-WPAN), the recent set of technologies conforming the so-called Low-Power Wide Area Networks (LP-WAN) offers long-range communications, allowing one to send small pieces of information at a reduced energy cost, which promotes the creation of new IoT applications and services. However, LP-WAN technologies pose new challenges since they have strong limitations in the available bandwidth. In general, a first step prior to a smart object being able to gain access to the network is the process of network access authentication. It involves authentication, authorization and key management operations. This process is of vital importance for operators to control network resources. However, proposals for managing network access authentication in LP-WAN are tailored to the specifics of each technology, which could introduce interoperability problems in the future. In this sense, little effort has been put so far into providing a wireless-independent solution for network access authentication in the area of LP-WAN. To fill this gap, we propose a service named Low-Overhead CoAP-EAP (LO-CoAP-EAP), which is based on previous work designed for LR-WPAN. LO-CoAP-EAP integrates the use of Authentication, Authorization and Accounting (AAA) infrastructures and the Extensible Authentication Protocol (EAP) protocol. For this integration, we use the Constrained Application Protocol (CoAP) to design a network authentication service independent of the type of LP-WAN technology. LO-CoAP-EAP represents a trade-off between flexibility, wireless technology independence, scalability and performance in LP-WAN.
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sensors
Article
A CoAP-Based Network Access Authentication
Service for Low-Power Wide Area Networks:
LO-CoAP-EAP
Dan Garcia-Carrillo 1,*, Rafael Marin-Lopez 1, Arunprabhu Kandasamy 2,3 and Alexander Pelov 2
1Department Information and Communication Engineering (DIIC), Faculty of Computer Science,
University of Murcia, 30100 Murcia, Spain; rafa@um.es
2Acklio, 2 BIS rue de la Chataigneraie, 35576 Cesson-Sevigne, France; arun@ackl.io (A.K.); a@ackl.io (A.P.)
3Institut MINES TELECOM, TELECOM Bretagne, 2 rue de la Chataigneraie CS 17607,
35576 Cesson-Sevigne CEDEX, France
*Correspondence: dan.garcia@um.es; Tel.: +34-868-887-882
Received: 26 September 2017; Accepted: 13 November 2017; Published: 17 November 2017
Abstract:
The Internet-of-Things (IoT) landscape is expanding with new radio technologies.
In addition to the Low-Rate Wireless Personal Area Network (LR-WPAN), the recent set of
technologies conforming the so-called Low-Power Wide Area Networks (LP-WAN) offers long-range
communications, allowing one to send small pieces of information at a reduced energy cost, which
promotes the creation of new IoT applications and services. However, LP-WAN technologies pose
new challenges since they have strong limitations in the available bandwidth. In general, a first
step prior to a smart object being able to gain access to the network is the process of network access
authentication. It involves authentication, authorization and key management operations. This
process is of vital importance for operators to control network resources. However, proposals for
managing network access authentication in LP-WAN are tailored to the specifics of each technology,
which could introduce interoperability problems in the future. In this sense, little effort has been
put so far into providing a wireless-independent solution for network access authentication in
the area of LP-WAN. To fill this gap, we propose a service named Low-Overhead CoAP-EAP
(LO-CoAP-EAP), which is based on previous work designed for LR-WPAN. LO-CoAP-EAP
integrates the use of Authentication, Authorization and Accounting (AAA) infrastructures and
the Extensible Authentication Protocol (EAP) protocol. For this integration, we use the Constrained
Application Protocol (CoAP) to design a network authentication service independent of the type of
LP-WAN technology. LO-CoAP-EAP represents a trade-off between flexibility, wireless technology
independence, scalability and performance in LP-WAN.
Keywords: lightweight; network access authentication; IoT; LP-WAN; CoAP; EAP; AAA
1. Introduction
In the last few years, the global information network consisting of Internet-connected objects
known as the Internet of Things (IoT) [
?
] has experienced an important growth. Several wireless
technologies, such as Bluetooth [
?
] or IEEE 802.15.4 [
?
], give support to the myriad of devices
connected to IoT networks. These wireless technologies typically offer a short-medium range coverage
and allow the creation of Low-Rate Wireless Personal Area Networks (LR-WPAN). In addition to the
aforementioned wireless technologies, there has been a recent impulse toward Low-Power Wide Area
Networks (LP-WAN) [
?
]. LP-WAN encompasses wireless technologies for long-range communications.
LP-WAN allows one to send small pieces of information between the IoT devices and an antenna up to
several kilometers with small energy consumption, expanding the impact of IoT applications to a larger
Sensors 2017,17, 2646; doi:10.3390/s17112646 www.mdpi.com/journal/sensors
Sensors 2017,17, 2646 2 of ??
scale. Among such applications, we can mention the connection of alarms to the cloud, smart farming
and precision agriculture [
?
]. As a downside, LP-WAN only supports data rates as low as 50 b/s
or 250 kB/s, frame sizes as low as eight bytes and restrictions to access the medium with a duty cycle
of 0.1% to 10% on some Industrial, Scientific and Medical (ISM) bands [
?
]. In other words, apart
from the constraints observed in LR-WPAN, LP-WAN adds a very constrained communication link
(beyond those observed in LR-WPAN). Due to the concept around LP-WAN being promising, some
research groups and alliances (e.g., LoRaWAN [
?
], Sigfox [
?
], Developers’ Alliance for Standards
Harmonization of ISO 18000-7 (DASH7)-7 [
?
], Wireless Smart Ubiquitous Network (WI-SUN) [
?
],
etc.) are putting their resources into defining their own specific solutions to cover the downsides
of the LP-WAN networks quickly. Very recently, the Internet Engineering Task Force (IETF) has
created a Low-Power Wide Area Networks (lpwan) Working Group to determine how they can
contribute to the evolution of LP-WAN by defining technology independent protocols and solutions
that favor interoperability between the different wireless technologies involved in LP-WAN. For
example, modifications to the 6LoWPAN standard are under consideration to adapt IPv6 to LP-WAN
[
?
]. Another example is the adaptation of the Constrained Application Protocol (CoAP) [
?
] to
LP-WAN. CoAP is a specialized web transfer protocol for constrained nodes and networks designed
to implement different types of services (e.g., sensor readings, remote configuration, etc.) in IoT.
While there is an effort for homogenizing communications in this type of network based on IPv6 and
CoAP, little has been done to design a unifying solution, providing a common and well-defined model
for network access authentication for LP-WAN networks, independent of the wireless technology.
Nevertheless, this process, which involves authentication, authorization and key management, is also
fundamental since it allows operators and network administrators to decide whether a particular
device can or cannot join the network.
In this sense, the LPWAN WG has discussed the use of the Authentication Authorization and
Accounting (AAA) infrastructures to support network access authentication [
? ? ? ?
]. The reason is that
LP-WAN networks look very similar to current cellular deployments, but with very limited resources.
In fact, current wireless networks such as 3G, WiMAX or WiFi use, traditionally, AAA infrastructures to
control the access to the network service at a large scale [
???
]. These infrastructures have been based
on two main protocols: Remote Authentication Dial-In User Service (RADIUS) [
?
] or Diameter [
?
]. In conjunction with AAA, the Extensible Authentication Protocol (EAP) [
?
] is also considered to
authenticate the devices because it provides a wireless technology independent protocol for flexible
authentication and robust key management [
?
]. By definition, the EAP key management framework
allows obtaining key material to derive new key material to bootstrap a security association in a
concrete wireless technology.
In this paper, we propose a first solution for network access authentication in LP-WAN called
Low-Power CoAP-EAP (LO-CoAP-EAP) based on three pillars: AAA, EAP and CoAP. The use
of AAA infrastructures provides scalability and roaming for the network access authentication;
EAP allows different smart objects and organizations to use different authentication mechanisms
(based on symmetric keys, certificates, etc.) depending on their requirements; and CoAP, which is used
as a constrained transport for EAP between the smart object and the network, both connected through
a link with a very limited bandwidth. The general design of LO-CoAP-EAP starts from our previous
work in [
?
], named CoAP-EAP, but is redesigned to take into account the constraints in LP-WAN.
In particular, LO-CoAP-EAP provides a CoAP-based service for network access authentication in
LP-WAN, which promotes homogeneity and independence between different wireless technologies.
The new design of LO-CoAP-EAP reduces the number of exchanges and the overall number of bytes
sent. As a collateral effect, we will also prove that LO-CoAP-EAP substantially improves CoAP-EAP
in LR-WPAN networks.
The rest of this paper is organized as follows. Section
??
gives the background in order to
understand our solution. Section
??
details the related work. Section
??
details the proposed network
access authentication service, LO-CoAP-EAP: the architecture and operation. In Section
??
, we show
Sensors 2017,17, 2646 3 of ??
a performance evaluation including message length, authentication time, the proportion of successful
authentications (success ratio) and the energy consumption in two test-beds: (1) a real deployment of
a LP-WAN network based on LoRa technology to get in-the-field measurements; (2) a network based
on 6LoWPAN using the Cooja simulator to also show the impact in LR-WPAN networks. With these
two test-beds, we have compared LO-CoAP-EAP against existing related work. Finally, we provide
some conclusions and future work lines in Section ??.
2. Background
2.1. Authentication, Authorization and Accounting Framework
The Authentication, Authorization and Accounting (AAA) Framework [
?
] supports three basic
security services used in network deployments: authentication, to establish the identity of the end user;
authorization, describing the conditions under which the user is able to access the network resources;
accounting, tracking the resources consumed by the user. The AAA framework defines a model
consisting of an End User (EU) who intends to gain access to some specific network service, an Identity
Provider (IdP) that stores the identity of the EU and some long-term credentials; and a Service Provider
(SP) that manages the access to the network service. In a non-federated scenario, the IdP and the
SP belong to the same organization (the IdP’s organization). In a federated scenario, where the IdP
and SP belong to different organizations, some bilateral agreements are assumed among the domains
conforming the federation. The participant organizations in the federation will have independent
AAA servers that will be able to communicate and exchange AAA information among them using
an AAA protocol. The most commonly deployed AAA protocols are Diameter [
?
], widely deployed
in 3G networks, and RADIUS [?], used in WiFi and WiMAX.
The SP operates the Network Access Server (NAS) that communicates with the user (e.g., a smart
object) and the AAA infrastructure. In the most simple scenario, an AAA infrastructure consists of
a NAS, with a direct connection to the AAA server. In more complex scenarios, additional AAA
servers (called AAA proxies) can be deployed between the NAS and the AAA server for scalability or
to support federated access.
2.2. Extensible Authentication Protocol
The Extensible Authentication Protocol (EAP) [
?
] is a standard protocol for authentication that
allows the execution of authentication mechanisms (e.g., based on digital certificates, symmetric keys,
etc.), named EAP methods, without changing the protocol. As an example of the EAP method, we can
mention EAP-PSK [
?
], which provides a lightweight authentication mechanism based on Pre-Shared
Key (PSK). Other examples of EAP methods, such as EAP-Transport Layer Security (EAP-TLS) , can be
seen in [?].
EAP has been designed with the principle of media independence. That is, the protocol is
independent of the wireless technology used. A different matter is that EAP is not independent of
the link-layer and that is why the characteristics of a protocol that intends to transport EAP messages
(EAP lower layer) is specified in EAP.
By definition, EAP is a lock-step protocol, which means it handles a single packet, a request or
a response, per flight. Each EAP request is answered with an EAP response. The number of message
exchanges depends on the EAP method used. This gives flexibility to choose the authentication that
fits best in each case. Every EAP method is run between the EAP peer and the EAP server through
the EAP authenticator acting as a forwarder. To start an EAP authentication, the EAP authenticator
typically sends an EAP request/identity message to the EAP peer, whom in turn answers with
its identity. The identity is sent following the Network Access Identifier (NAI) format [
?
] (e.g.,
smartobject@homedomain). The NAI contains the smart object identity, separating the domain name
(homedomain) with an @ sign. Once the EAP server receives the identity of the EAP peer, it is able to
Sensors 2017,17, 2646 4 of ??
select the EAP method to be performed. The EAP method is performed using EAP request/responses
between the EAP server and the EAP peer.
There are two models to deploy EAP. On the one hand, we have the EAP standalone model,
in which the EAP authenticator and the EAP server are co-located in the same device. This model can
be easily used in small deployments, where no AAA is required. On the other hand, when scalability
becomes a must (as happens in the context of this paper), the EAP pass-through authenticator model
is used. In this case, the communication between the EAP pass-through authenticator and the EAP
server is done using an AAA protocol. Common to both cases is the use of an EAP lower layer to carry
EAP messages between the EAP peer and the EAP authenticator. In this paper, we have designed a
CoAP-based EAP lower layer and, therefore, independent of the EAP method in use.
One important feature to take into account is that some EAP methods [
?
] generate key material.
According to the EAP Key Management Framework (KMF) [
?
], two keys are exported after a successful
EAP authentication: the Master Session Key (MSK) and the Extended Master Session Key (EMSK).
From both keys, only the MSK has a defined use for network access authentication in order to run a
Security Association Protocol (SAP) to derive Transient Session Keys (TSK). In turn, the TSKs allow
one to protect the communications between the EAP peer and EAP authenticator. The MSK is sent
by the AAA server to the EAP authenticator using the AAA protocol, while the EMSK must not be
provided to any other entity, keeping it only between the EAP peer and the EAP server.
2.3. Constrained Application Protocol
CoAP is defined in [
?
] as a web transfer protocol specifically designed to be used with constrained
devices and constrained networks in the IoT. The purpose of CoAP is to realize a subset of REST
optimized for M2M applications. In fact, it has some built-in capabilities designed for M2M such as
discovery of services and capabilities, support for multicast and asynchronous message exchanges,
with a very low overhead and simplicity, having constrained environments in mind. In the specific
case of LP-WAN, CoAP presents a proper alternative where communication between devices has to be
performed in a standardized manner using a RESTful protocol, with low overhead. Recently, a work
has been proposed to further reduce the overhead of CoAP by Minaburo et al. [?].
CoAP distinguishes three different categories of messages: request, response and empty.
A message can be of four different types: Confirmable (CON), Non-confirmable (NON),
Acknowledgment (ACK) and Reset (RST). Unlike NON messages, CON messages require the reception
of an ACK. RST is used when the received message (CON or NON) cannot be processed. CoAP
messages contain a code field, which gives semantics to the message, whether it is a request or a
response, and if the response denotes success or failure of some kind. There is another field called
token, which is used to match requests with responses. Additionally, CoAP supports the addition of
a sequence of zero or more CoAP Options in the Type-Length-Value (TLV) format. CoAP requests
contain different methods (or actions over the resource): GET, POST, PUT and DELETE. GET obtains
the current content of the resource identified by the URI. POST messages allow for the client and
server to exchange information and, according to the context of the information, to process the request
accordingly. PUT is used to create or update the resource identified by the URI. Finally, DELETE
requests that the resource represented by the URI has to be deleted.
3. Related Work
Network access authentication for IoT is described in Heer et al. [
?
] and Garcia-Morchon et al. [
?
] as part of the process of a smart object joining an IoT network securely, at a specific time and place.
This process entails the authentication and authorization of the smart object, as well as the transfer of
security parameters (e.g., key material) for a trustworthy operation in the security domain.
In general, although most of the current work for network access authentication in IoT has been
devoted to LR-WPAN, some of these (specified below) deserve attention due to the relation to our
solution. Large-scale scenarios use AAA infrastructures and EAP, while small to medium scale do not
Sensors 2017,17, 2646 5 of ??
involve AAA infrastructures. For example, Garcia-Morchon et al. [
?
] in the case of the centralized
scenario point out EAP as a candidate to perform the authentication and generation of key material,
proposing the Protocol for carrying Authentication for Network Access (PANA) [
?
] as a standard and
link-layer independent EAP lower layer. In fact, the ZigBee IP standard [
?
] uses PANA as a protocol
for network access authentication. As such, our proposal LO-CoAP-EAP can be compared with PANA,
as we will analyze in Section
??
. O’Flynn et al. [
?
] consider also a centralized architecture using EAP
and PANA or 802.1X [
?
] as EAP lower layers, while Sarikaya [
?
] and Sarikaya et al. [
?
] propose
EAP-TLS concretely as the authentication protocol, but without interaction with an AAA infrastructure.
S. Das et al. [
?
] also propose a centralized alternative using PANA, EAP and AAA to bootstrap a PSK
to establish a Datagram Transport Layer Security (DTLS) [
?
] or Internet Key Exchange (IKEv2) [
?
]
security association between the smart object and the PANA Agent (PAA), using afterwards CoAP
for the normal operation. Alternatively, Moreno et al. [
?
] designed and implemented a lightweight
version of a PANA client (PaC) for Contiki O.S. [
?
] (PANATIKI) by adapting PANA for constrained
devices. Finally, Garcia-Carrillo and Marin-Lopez [
?
] proposes a technology independent EAP lower
layer using CoAP to transport EAP messages (CoAP-EAP) that improves the PANA-based solutions,
to leverage the features of CoAP as a suitable protocol for communication between constrained devices
and use of AAA infrastructures to provide scalability and federation capabilities.
There are also solutions that use EAP adapted to IoT, by either modifying EAP itself, or defining
new EAP methods more optimized for the constraints of IoT networks. However, these solutions
imply that wireless technologies where EAP is applied need to define an EAP lower layer specific to
the technology. For example, it would imply that existing LP-WAN technologies should design their
own link-layer EAP lower layers to transport those EAP methods, which is far from the general case. On the
contrary, LO-CoAP-EAP offers a wireless independent EAP lower layer to avoid this problem and allows
existing or new EAP methods for IoT to be used without modifying the underlying technology.
Thus, the constraint LP-WAN imposes on the network link requires adapting or redesigning
existing solutions. Most of the LP-WAN technologies are fairly recent and are still under development,
and as a consequence, solutions related to security and network access authentication are still
immature. Specifically, some of the alliances and organizations working in the area of LP-WAN
propose solutions to authenticate and secure the communications using pre-shared keys. Additionally,
the key management is usually handled in a static manner, with no dynamic key distribution, and
there is little information about how the necessary keys will be managed, installed, refreshed, etc., or
how the devices will be authenticated to access the network and how they will interact securely with
the infrastructure. For example, DASH-7 [
?
] includes the notion of authentication and access control,
defining three users (root, user and guest) and authentication keys root and user, as well as a network
key. There is no specific mention about the method used to generate the aforementioned keys, if they
are configured manually, or provided dynamically using other methods of key management. Sigfox [
?
]
provides integrity of the communications, with encryption at the application level, and it considers the
future integration of a secure element to store key material. However, there is no mention of how these
keys are installed, either manually or through dynamic key management methods. IEEE 802.11ah [
?
]
defines a modification in the scheduling of the network access authentication over the IEEE 802.11ai
amendment, but no references to the modifications of the network access authentication methods
are made, so we consider that they are the same as defined for IEEE 802.11. LoRaWAN [
?
] does
include a join procedure to authenticate the IoT devices based on pre-shared key Application Session
Key (AppSKey) to derive fresh keys (NetworkSKey and AppSKey) to protect the LoRa link between
the IoT device and the network. Recent work ([
?
] and Diameter [
?
]) has defined a way to integrate the
LoRaWAN join procedure with AAA infrastructures to provide scalability and roaming support. However,
these solutions are tailored to LoRaWAN.
Table
??
summarizes the current state of several technologies related to LP-WAN security and
network access authentication, showing the algorithms used to provide integrity and encrypt the
messages and if the technology provides a key management protocol to derive fresh key material to
Sensors 2017,17, 2646 6 of ??
protect the link. There is a common occurrence of the use of symmetric cryptography, understandable
due to its properties, providing security at a computationally low cost in comparison with asymmetric
cryptography, and furthermore, the existence of hardware implementations of the most common
crypto suite, Advanced Encryption Standard (AES), increases its efficiency.
Table 1. Crypto suites used by LP-WAN technologies to authenticate and encrypt the frames.
Technology Authentication/ Payload Encryption Keyl
Integrity Algorithm Algorithm Management Protocol
LoRaWAN AES-128 Cipher-based Message Authentication Code (CMAC) AES-128 YES
Sigfox N/A N/A NO
IEEE802.15.4k N/A N/A -
IEEE802.15.4g AES-CCM AES-CCM -
Random Phase Multiple Access (RPMA) Twoway16byte-hash 256-bit -
DASH-7 AES Cipher Block Chaining(CBC)-MAC-128/64/32 AES Counter Mode (CTR) -
AES-CCM-128/64/32
Weightless AES-128/256 AES-128/256 YES
NB-LTE Authentication and Key Agreement (AKA)(128 bit keys) AKA (128 bit keys) YES
Thus, to the best of our knowledge, this paper is the first work to contribute a solution for network
access authentication for LP-WAN that tries to deal with the process in a homogeneous way through
different technologies.
4. Network Access Authentication in LP-WAN: LO-CoAP-EAP
4.1. Requirements of the Service
The general requirement that leads to the design of the LO-CoAP-EAP service is to have a network
access authentication service for LP-WAN regardless of the underlying radio technology being used,
relaying on current standards, with the following characteristics:
Flexible authentication mechanism: The service has to provide the needed flexibility for the
authentication due to the variety of operators, technologies and requirements of each deployment.
Operational homogeneity: The service had to be built on top of a protocol that is common to most
of the IoT devices.
Link-layer independent solution: The solution should be independent of the link-layer technology
to be supported by the set of LP-WAN technologies.
Reduce overhead: Due to the high restrictions of LP-WAN in terms of bandwidth, we need
a solution with a reduced overhead, saving as much messages and bytes sent over the link
as possible.
Bootstrapping subsequent security association protocols: As each LP-WAN technology may
rely on different protocols to secure its data communications, we need to provide key material
for different protocols to secure subsequent communications between the smart object and
the network.
4.2. The LO-CoAP-EAP Service
Low-Overhead CoAP-EAP (LO-CoAP-EAP) is a CoAP-based and link-layer independent
network access authentication service designed considering the constraints imposed by LP-WAN
networks. LO-CoAP-EAP builds on top of three main technologies: CoAP, AAA and EAP. We use
CoAP as a lightweight application protocol for constrained devices, allowing reusing source code
in existing smart objects since CoAP is a common piece in IoT stacks for Machine-to-Machine
(M2M) communications [
?
]. The integration with AAA infrastructures provides scalability,
roaming/federation support and a common core independent of the technology to centralize the
authentication and key distribution procedures. With EAP, we have flexibility to choose the
authentication method, depending on the requirements of different organizations. Additionally,
Sensors 2017,17, 2646 7 of ??
the EAP Key Management Framework (EAP KMF) provides the rules for key derivation to bootstrap
security associations for different technologies, to protect data traffic in the constrained link.
Next, we describe the architecture of LO-CoAP-EAP network authentication service and its
operation, and we detail the main changes with respect to our previous work in order to adapt the
solution to cover LP-WAN networks.
4.3. Architecture
LO-CoAP-EAP defines three main entities in its general architecture: the smart object,
the controller and the AAA server. The smart object is the target of the authentication, and the
controller manages the network. It offers services to the security domain and intermediates during
network access authentication between the smart object and the AAA server. Finally, the AAA server
is in charge of the authentication and granting permissions for the requested services. LO-CoAP-EAP
entities integrate EAP, CoAP and AAA as follows: the smart object instantiates an EAP peer and a CoAP
client and server; the controller integrates an EAP authenticator, a CoAP server and client and an AAA
client to interact with the AAA server (authentication server) of the identity provider. It is worth noting that
there can be one or more AAA servers between the controller and the authentication server.
The Internet Engineering Task Force (IETF)LPWAN WG defines in its overview document [
?
]
a generic terminology for the architecture. They define end-devices as the things, sensors, actuators,
etc. The radio gateway is the end-point of the constrained link. The network gateway (or network
server) is the interconnection between the radio gateway and the Internet. Finally, LPWAN-AAA is
the entity in charge of managing the authentication. The LO-CoAP-EAP architecture maps to their
LP-WAN counterparts as follows: the smart object is the end-device; the controller acts as the network
server; and the AAA server as the LPWAN-AAA.
4.4. General Operation
To refer to the CoAP-based service for network authentication, LO-CoAP-EAP uses the URI
coap://
<
controller-IP
>
/b. The smart object, acting as the CoAP client, initiates the LO-CoAP-EAP
exchange. Without loss of generality, we use RADIUS as the AAA protocol in the description.
The operation, shown in Figure ??, is as follows:
The smart object sends a POST message (Step 1) with the no-response option [
?
]. This option
allows the smart object to signal it does not expect a response to this request. In this first message,
the smart object sends a random number nonce-s carried in an option named nonce and the smart
object’s identity in NAI format [
?
] (user@domain.org) in the payload. After this first message, all the
remaining exchanges will be done with the controller acting as the CoAP client and the smart object
as the CoAP server. This role choice follows the guidelines in [
?
] to simplify the implementation
assuming that the smart object will be more constrained than the controller. The controller can now
process the smart object’ identity and send it to the AAA server in a RADIUS access-request (Step 2).
Typically, the identity is transported in a EAP response/identity, but this implies sending an EAP
request/identity first. To save this exchange, the RADIUS (and Diameter) standard allows the smart
object’s identity to be carried into an attribute called user-name instead of EAP-response/identity
message. In this case, the attribute EAP-message, which contains EAP messages, will be empty in this
message [
? ?
]. Additionally, it will include a NAS-port-type. This attribute indicates the physical port
of the controller that is authenticating the user, and it is useful for the AAA server to know whether
the access is being performed over an LP-WAN link, which may modulate the authorization decision
and the delivery of configuration parameters for the controller. After the AAA server processes this
message, it decides what EAP method is to be used based on the smart object’ identity; let us call
it EAP-X. Then, it responds with the first message of the EAP method (EAP-X 1) embedded in the
attribute EAP-message of a RADIUS access challenge (Step 3). When this message arrives at the
controller (Step 4), it decapsulates EAP-X 1, encapsulating it in the payload of a confirmable POST
message. The token value in CoAP is set to empty to further reduce the length of the message. The
Sensors 2017,17, 2646 8 of ??
smart object answers this POST with a piggybacked response that contains the EAP response (EAP-X 2)
(Step 5). The controller will then forward EAP-X 2 to the AAA server using a RADIUS access-challenge
(Step 6), and so on. This process continues until the EAP method finishes (Steps 7–10), the number of
exchanges depending on the EAP method. Finally, the AAA server sends a RADIUS access-accept with
an EAP success. Along with the EAP success, the MSK is delivered to the controller with a network
access lifetime (i.e., session-timeout) (Step 11). Then, the controller sends a confirmable POST message
that contains a nonce nonce-c in the nonce option and authorization data like the session lifetime
(Step 12) in the payload.
Smart&Object
POST /b [NON, M ID=0,Token( Empty),
Options(nonce-s, no-re sponse)],
Payload (mo te @u m.es )]
POST /b [CON, M ID=3,Token( Empty)],
Payload (EAP-X 1)
ACK [MID=3, Token(Empty ), 2.01 Created (b/x)],
Payload (EAP-X 2)
POST /b/x [CON, MID=4, Token(Empty) ],
Payload (EAP-X n-1 )
ACK [MID=4, Token(Empty ), 2.04 Chan ged],
Payload (EAP-X n)
POST /b/x [CON, MID=5,Token(Empty),
Options(URI-Host, nonce-c, AUTH) ], Payl oad(Au thZ)
MSK ACK [MID=5, Token (Empty), 2.04 Changed,
Options(AUTH)]
Access Request( EAP-Message(Empt y),
User-Name: mote @u m.es ,
NAS-Port-Ty pe :1 8,
Servic e-Type : bootstrapping)
Access Challeng e (EAP-X 1)
MSK
1
4
5
8
9
12
13
2
3
6
7
10
11
Con tro ller AAA& Se rver
Access Challeng e (EAP-X 2)
Access Challeng e (EAP-X (n-1) )
Access Challeng e (EAP-X (n) )
Access Accept(EAP Success,
Sess ion -Timeout, MSK)
.
.
.
Phase I
Phase IV
Phase V
Figure 1.
Low-Overhead (LO)-CoAP-EAP bootstrapping service flow using any EAP method
X (EAP-X).
Typically, the EAP success is also included in this message. However, we do not include it in order
to save bandwidth, since the EAP standard allows the EAP peer (smart object) to proceed without
receiving the EAP success, but with an alternate indication of success [
?
]. This indication happens
in two ways: (1) the reception of the confirmation POST message without EAP and with the AUTH
option is an indication that the controller considers the EAP authentication finished. Second, the smart
object knows that the EAP authentication went well if an MSK is available. Nevertheless, both entities
still need to prove the possession of the MSK as mentioned in the EAP Key Management Framework
(EAP KMF). It is worth noting that this last exchange (12–13) is integrity protected by an authentication
tag embedded in an AUTH option. This provides a simple way of providing proof-of-possession of
the MSK between the smart object and the controller.
To protect the communications between the smart object and the controller, after the
LO-CoAP-EAP authentication service is completed, our solution is open to run the Security Association
Protocol (SAP) for any particular LP-WAN technology, providing fresh, shared key material with the
key derivation capabilities of the EAP KMF, as we elaborate in the next section.
The reader will note that there is a division of the exchanges in different phases in Figure
??
.
This division is there to compare easily the changes done over CoAP-EAP elaborated in Section
??
,
where we describe each phase and its function.
Sensors 2017,17, 2646 9 of ??
4.5. Bootstrapping Security Associations for LP-WAN
In this section, we specify how to derive key material from the MSK to secure the communications
between the controller and the smart object. The derived keys are known as Transient Session Keys
(TSKs) [
?
] in EAP lingo. Based on these keys, we can run virtually any security association that relies
on pre-shared keys (in fact, existing wireless technologies such as WiFi or WiMAX already derive TSKs
from the MSK). To illustrate this, we provide a simple example of this procedure based on LoRaWAN [
?
]. In particular, LoRaWAN requires the AppKey to run its security association protocol that involves
two messages: join request/join response. Once the EAP authentication is successful, both the smart
object and the controller share the MSK. From the MSK, we derive a TSK, which will be the AppKey
in the LoRaWAN specification. For the derivation process, we use a similar Key Derivation Function
(KDF) as the one specified in [
?
]. Specifically, we use AES Cipher-based Message Authentication Code
(CMAC)-PRF-128 as the Pseudo Random Function (PRF), which uses AES-CMAC-128 as a primitive.
Both primitives use AES-128 as the building block since it is widely used in constrained devices. Then,
as the Key Derivation Function (KDF) we use the function PRF+ defined in [
?
], as recommended in
[
?
]. The PRF+ is able to generate key material of different lengths. The example of the derivation of
the AppKey can be seen in Equation (
??
). The AppKey is a 16-byte length key. The input for the KDF
(PRF+) is the following: the MSK derived from the EAP method; an ASCII code representation of the
non-NULL terminated string “IETF_LoRaWAN” (excluding the quotes), to which we concatenate the
NULL value and the nonces exchanged. Sixty four is the length of the MSK;
length
is the length of the
output (16 in the case of the AppKey).
AppKey =KDF(MSK, “I ETF_LoRaWAN00 |NULL |nonce c|nonce s, 64, length)(1)
The same KDF can be used to generate key material for any other technology that requires key material
to protect data frames. The unique changes would be the replacement of the label “IETF_LoRaWAN” for
another more appropriate one (e.g., IETF_SigFox) and the expected length of the key material.
As depicted in Figure
??
, once both entities have derived the AppKey, they can simply run the
LoRaWAN
OTA security association protocol that allows one to derive two keys, Application Session
key (AppSKey) and Network Session Key (NwkSKey), to protect communications (see more details
in [?]).
Smart&Object
Join Request
Join Accept
POST /b/x [CON, MID=5,Token(Empty),
Options(URI-Ho st, nonce-c, AUTH) ], Payload(Au thZ )
MSK
ACK [MID=5, Token (Empty), 2.04 Changed,
Options(AUT H)]
MSK
14
15
12
13
Con tro ller
----------------- ( Auth Completed ) ------------------
MSK
AppKe
y
AppSKey
NwkSKey
AppSKey
NwkSKey
Figure 2. LoRaWAN security association establishment after LO-CoAP-EAP authentication.
Sensors 2017,17, 2646 10 of ??
4.6. Main Changes to CoAP-EAP
For the design of LO-CoAP-EAP, we perform a set of changes to CoAP-EAP to achieve the primary
target of this paper: reduce the number of bytes dedicated to the network access authentication process
traveling over the constrained link. The result is a reduction of the number of messages on flight and
the size of the messages exchanged between the smart object and the controller.
To achieve this, we have analyzed each phase of CoAP-EAP in order to determine which parts
can be simplified. In this sense, CoAP-EAP can be divided into five phases: (I) trigger; (II) exchange of
nonces; (III) exchange of identity; (IV) exchange of EAP method; (V) sending the EAP success. Next,
we see how LO-CoAP-EAP deals with each phase.
Figure
??
illustrates the changes in CoAP-EAP. The red (not highlighted) arrows signify that those
messages are deleted in LO-CoAP-EAP, either because of being simplified or that information moved
to other messages. The content in the messages is crossed out and colored in red, signifying that the
content has been simplified. Common simplifications to all phases are: (1) the URI to identify the
authentication service is reduced from /boot to /b to save three bytes in each request; (2) the token
previously generated randomly, fixed for the duration of the exchange and used as session identifier is
now set to empty. Discussion about the implications in LO-CoAP-EAP is shown in Section ??.
Smart&Object
POST /boot [CON, MID=0,Token(0x01),
Options(nonce-s, no-respo nse)], Payload(mote@um.es)]]
ACK [MI D=0, Token (0x01), 2.04 Changed, Payload(“OK”)]
POST /boot [CON, MID=4, Token (0x01), Payload(nonce-c)]
ACK [MID=4, Token (0x01), 2.01 Created,
Location-Pa th( boot/5) Payload(nonce-s) ]
POST /bo ot/5 [CON, MID=10, Token(0x01),
Payload(EAP Req/Id)]
ACK [MID=10, Token (0x01), 2.04 Changed,
Payload(EAP Rep/Id) ]
POST /boot/5 [CON, MID=11,Tok en( 0x01),
Payload (EAP-X 1)]
Access Request (EAP(Rep/Id),
User-Name: mote @u m.es,
NAS-Port-Ty pe :1 8,
Service-Ty pe : bootstrapping)
Access Challeng e (EAP-X 1)
MSK
1
2
3
4
5
6
98
12
7
Con tro ller AAA& S erve r
ACK [MID=11, Tok en (0x01),2.04 Changed,
Payload (EAP-X 2) ]
POST /boot/5 [CON, MID=14, Token (0x 01),
Payload (EAP-X (n-1))]
ACK [MID=14, Token (0x01), 2.04 Changed,
Payload (EAP-X n ) ]
POST /boot/5 [CON, MID=19,Toke n(0x 01 ),
Options(Session-Timeout*, nonce-c,AUTH*)
Payload (EAP Succe ss)]
ACK [MID=19, Token (0x01), 2.04 Changed, AUTH* ]
Access Request (EAP-X 2)
Access Challeng e (EAP-X (n-1)
)
11
Access Request (EAP-X n )
Access Acce pt(EAP Success,
MSK, Session-Timeout)
10
13
14
17
18
16
15
MSK
CoAP RADIUS
.
.
.
Phase I
Phase II
Phase III
Phase IV
Phase V
Figure 3. Changes to the original CoAP-EAP.
Trigger (Phase I): To start the process in LO-CoAP-EAP, we send in the first POST used to trigger
the authentication process the identity of the user in the payload of the message and the nonce-s in
a new option (nonce option) for the controller. Additionally, this message carries a no-response option [
?
], which indicates to the controller that there is no need for a response of any kind to this request.
Sensors 2017,17, 2646 11 of ??
Exchange of the nonces (Phase II): The nonce exchange that was previously done following the
trigger message is avoided, and the nonces are embedded in other messages. Although this saves
a complete exchange, its original purpose is still considered to alleviate Denial of Service (DoS)
attacks, as is discussed further in Section ??.
Exchange of the Identity (Phase III): After the nonce exchange, the EAP identity was requested.
This exchange is now avoided, since the EAP standard specifies this exchange as optional.
The identity of the user, as mentioned previously, is now sent in the trigger message. With this
change, we save another exchange.
EAP method exchange (Phase IV): The messages involved in the exchange of EAP method exchange
are not further altered beyond the simplifications mentioned before and run as expected.
Sending the EAP success (Phase V): Sending the EAP success message to the smart object is also
optional. Avoiding sending the EAP success in the last exchange, we save a four bytes. We only
send the nonce-c to the smart object for key derivation purposes in that last request.
It is worth noting that, as in any type of authentication process, we need radio communication
between the smart object and the controller for the LO-CoAP-EAP service to work. Additionally,
communication between the controller and the AAA infrastructure is also needed to complete the
process. This communication is done through the WAN connection, not subject to the bandwidth
limitations. Certainly, the EAP exchanges in the authentication involves an indirect communication
between the smart object and the AAA server using the controller as the intermediary, but this process
will only be done once; therefore, the smart object does not need constant connection with the AAA
infrastructure after the authentication.
4.7. Additional Discussion
4.7.1. Session Identifier and Empty Token
Changing the token value to empty saves some bytes sent over the link related to the authentication
service. However, we argue that this change does not affect the operation of the protocol.
Indeed, the purpose of the CoAP token is to correlate a CoAP request and response [
?
].
More specifically, it is intended as a client-local identifier to differentiate between concurrent requests.
Based on this, we state how the LO-CoAP-EAP protocol still maintains its functionality in spite of
setting its value to empty. Firstly, since EAP is a lock-step protocol (see Section
??
), the LO-CoAP-EAP
protocol that transports EAP is also designed as lock-step. The reason is simple: to obtain network
access through a specific controller, a single authentication process is enough. After all, the link is
very constrained, and any traffic should be reduced. Therefore, the controller (CoAP client) will not
send a new request until having received the corresponding response. Secondly, the original use of the
token (as the session identifier) can be fulfilled by other means: we only need a known value by both
parties that uniquely identifies the smart object. This can be done using the IPv6 address or the MAC
address in case we consider a direct link between the smart object and the controller.
4.7.2. Analyzing Retransmissions
Retransmissions are a tool to provide reliability for the communications, sending again a message
if there is no indication that the message arrived successfully. The retransmission policy suggested in
the CoAP standard has been adapted for LR-WPAN networks, though some improvements may
be still performed in the default policy [
?
]. In particular, the default policy specifies to wait
approximately 2 s between when a message is sent and the acknowledgment arrives and increasing
exponentially the retransmission time to double the previous retransmission time, up to a maximum
of four retransmissions.
Nonetheless, this retransmission policy needs further considerations in radio technologies with
a very low bit rate, such as LP-WAN networks. For example, according to LoRa technology, assuming
a Spreading Factor (SP) of 12, the bandwidth set to 125 kHz and the payload to the maximum allowed
Sensors 2017,17, 2646 12 of ??
in LoRa (256 bytes) and setting the other variables as default in the Semtech LoRa Modem Calculator
tool [
?
], a message will spend on the air approximately 7.5 s. As we can observe, before a CoAP
message arrives at the other endpoint, a retransmission may be triggered (in some cases, more than one).
Thus, it is necessary to adjust the retransmission policy in LPWAN networks.
We need to to assign the minimum elapsed time before a retransmission is sent. In CoAP, this is
called ACK_TIMEOUT. Therefore, we need to consider the parameters used in LoRa when transmitting
to get this number for the CoAP policy to establish its value. Basically, we need to know the Round
Trip Time (RTT) of a message (the time it takes to arrive from a source to a destination and back)
to establish ACK_TIMEOUT. Following the previous example and the implementation of the CoAP
retransmission mechanism, we would set the ACK_TIMEOUT to 8 s (rounding up to cover processing
time), which will give us a MAX_TRANSMIT_SPAN of 152 s, which corresponds with the time a
client considers that a confirmable message was not received. Considering the results of the previous
example, the controller will understand that a LO-CoAP-EAP authentication is discontinued after
waiting 152 s for a piggybacked response.
LoRa technology has adaptive capabilities that need to be taken into account when establishing
a retransmission policy for CoAP. As a side note, we comment that EAP has a retransmission
mechanism that is disabled, since it is running over a our reliable lower layer LO-CoAP-EAP.
Additionally, having a lower bit rate in the constrained link has an effect on the time each party
involved in LO-CoAP-EAP needs to store any state related to it. The smart object, and its EAP state
machine, needs to configure the amount of time to wait for a valid request before aborting (known
as ClientTimeout in [
?
]). It is also the case for the AAA server that needs to keep the status related
to the EAP authentication. If it is not stored for sufficient time, the AAA server might assume the
authentication has failed, erasing the associated state, when it is simply just taking longer.
4.7.3. DoS Attacks
The new design of LO-CoAP-EAP keeps the same security properties of CoAP-EAP discussed
in Section 3.6 in [
?
], except for one particular aspect: LO-CoAP-EAP may save two messages after
exchanging the smart object’s identity and use the smart object’s identity in the very first message
(Step 1 in Figure ??), so that the controller can contact the AAA server immediately.
Although the modifications in the first message save bits in the link, this creates an additional
state (authentication state) in the controller beyond just storing the smart object’s identity. In particular,
this authentication state includes the EAP state machine, which must be initialized with different
parameters [
?
], and the AAA client session required to communicate with the AAA server. This has
an important implication: an attacker may blindly send the “trigger” message with different MAC or
IPv6 addresses in a loop, therefore creating the authentication state for each trigger message sent to
the controller.
When a link has unrestricted bandwidth, the number of messages starting authentications arriving
at the controller may be very high. Each one will create some authentication state that the controller
has to store for a determined time. This would provoke growth in the number of authentication states
in the controller that surpass the capacity of the controller. However, if the capacity of the link is very
reduced, then the maximum number of messages starting an authentication is dramatically reduced,
as in the case of LP-WAN [
?
], limiting the authentication state generated in the controller. For example,
if we use LoRaWAN, when the devices behave respecting the duty cycle, we can expect less than one
authentication request per second. In the worst case, if an attacker were to use the channel at full
capacity, we can expect around sixteen messages per second, in each channel. A controller that has to
manage thousands of legitimate devices is assumed to be able to manage this amount of states created
by attackers. In any case, if the controller evaluates that it is creating states at an abnormal rate, it
can always perform the handshake to mitigate this effect. In summary, the link in LP-WAN is the
main bottleneck and most restricted resource, which limits the number of authentications per second
Sensors 2017,17, 2646 13 of ??
that can start in the controller. In fact, sending additional messages may create a problem in the link
regardless of the use of LO-CoAP-EAP.
Either way, in order to alleviate potential DoS attacks, the controller can always engage in
an optional (it was mandatory in CoAP-EAP) handshake (1a and 1b in Figure
??
) with the smart
object before creating the authentication state (EAP and AAA). In this manner, the attacker cannot
provoke the creation of the authentication state using a loop since it must answer correctly the message
sent by the controller. In any case, it is up to the controller’s policy to select when this handshake
should be performed or not (which is out of scope of this work). For example, the controller may
detect some irregular activity during the access (e.g., many triggers in a very short period of time) and,
as a consequence, activate this handshake to avoid consuming resources for the authentication state.
Figure 4. LO-CoAP-EAP handshake.
5. Experimental Results
To test different technologies and conditions, we have performed evaluations in the Cooja
simulator (Test-Bed 1) [
?
] and with real devices for LP-WAN (Test-Bed 2). The first testbed is
used to achieve three goals: (1) to get an approximation of the performance of the protocol from one to
several IP hops with different loss ratios, providing hard conditions in the link, since there is ongoing
work in LP-WAN exploring the multi-hop case [
???
]; (2) to compare LO-CoAP-EAP with PANATIKI [
?
], which is a PANA implementation (a current standard for a link-layer independent network access
authentication in IoT) adapted to the Contiki O.S.; and (3) to give proof that LO-CoAP-EAP also
provides important improvement with respect to CoAP-EAP in LR-WPAN. Based on the results of
this first approximation, we have prepared a real LP-WAN deployment using LoRa radio technology
(a LoRaFabian [
?
] network), as a representative example of LP-WAN, with real devices for LP-WAN.
Without loss of generality, we have obtained our results using a light and standard EAP method,
EAP-PSK. This method consists only of four messages to complete the authentication. Since our
solution is independent of the EAP method, any other method could have been used.
The parameters to be measured are: (1) message length, (2) network authentication time
and success ratio (that is, the relation between finished and initiated authentications), (3) energy
consumption and (4) memory footprint.
5.1. Message Length
In general, the message length is relevant in terms of the time the smart object takes to process it
(including the time to send and receive messages over the network). In LP-WAN, this is especially
relevant, taking into account the restrictions of LP-WAN in the link.
Table
??
shows the comparison in terms of message length (in bytes) for each alternative:
(1) PANATIKI, (2) CoAP-EAP, (3) LO-CoAP-EAP with handshake and (4) LO-CoAP-EAP without
handshake. We detail the length of the EAP lower layer without the EAP message (LL) and including
it (LL + EAP). PANATIKI is shown for reference since a detailed description of the message length
comparison with CoAP-EAP is done in [
?
]. This will give a better understanding of the impact the
Sensors 2017,17, 2646 14 of ??
redesign has on the reduction of the size of the protocol and, overall, the percentage of bytes saved in
each case.
Table 2. Comparison of the message lengths. PANATIKI, PANA implementation of Contiki.
PANATIKI CoAP-EAP LO-CoAP-EAP
with Handshake
LO-CoAP-EAP
without Handshake
Phase Message (CoAP-EAP) LL LL + EAP LL LL + EAP LL LL + EAP LL LL + EAP
I 1) POST 16 16 13 13 29 29 29 29
II 2) POST(nonce-c) 40 40 18 18 6 6 - -
3) ACK(nonce-s) 40 40 20 20 8 8 - -
III 4) POST(Request/Id) 43 48 16 21 - - - -
5) ACK(Reponse/Id) 41 60 9 28 - - - -
IV 6) POST(EAP-PSK 1) 27 56 16 45 9 38 7 36
7) ACK(EAP-PSK 2) 24 84 9 69 5 65 9 69
8) POST(EAP-PSK 3) 25 84 16 75 9 68 9 68
9) ACK(EAP-PSK 4) 25 68 9 52 5 48 5 48
V 10) POST(EAP success) 84 88 35 39 34 38 34 38
11) ACK 52 52 27 27 23 23 23 23
% Reduction over PANATIKI - - 55% 36% 69% 49% 72% 51%
% Reduction over CoAP-EAP - - - - 32% 20% 38% 23%
Total 417 636 188 407 128 323 116 311
LL: Lower Layer message length; LL + EAP: Lower Layer message length including EAP message length.
Overall, an important reduction in size of the lower layer can be appreciated. With CoAP-EAP
over PANATIKI, we reduce up to
50% comparing the lower layer and up to
32% the lower
layer + EAP messages. With LO-CoAP-EAP over PANATIKI, we reduce up to
70% compared with
the lower layer,and up to
50% the lower layer + EAP messages. Comparing LO-CoAP-EAP with
CoAP-EAP and with LO-CoAP-EAP with handshake, we have a reduction of
30% for the lower
layer and a reduction of
22% over the whole protocol exchange (lower layer + EAP messages).
For LO-CoAP-EAP with handshake, this reduction is
39% for the lower layer and
25% for the
whole protocol exchange compared with CoAP-EAP. This reduction is important to consider in very
constrained links such as LP-WAN networks, as we show in Section ??.
5.2. LO-CoAP-EAP Performance in the Cooja Simulator
Figure
??
shows the test-bed we use in the Cooja Network Simulator with Contiki OS Version 2.7 [
?
]. The smart objects used are the Zolertia Z1 with 92 kB of nominal ROM when compiled with 20-bit
architecture support and 8 kB of RAM. The compiler is msp430-gcc Version 4.7.2. The specifications of
the computer used for running the test-bed are shown in Table
??
. In terms of the software packages,
we have used cantcoap [
?
] ported to C language since it gives us the flexibility needed to only
create CoAP messages without including the REST engine integration, which is sufficient for our
proof-of-concept implementation. In this test-bed, we use the RPL border router, which enables
communication between the simulated Cooja Network and the outside physical network where the
controller is located. Between the border router and the smart object, there can be zero or more smart
objects. Having several hops between the smart object and the controller allow us to observe the
behavior of the network access authentication process in multi-hop networks and how each parameter
is affected (network authentication time, success ratio and energy consumption), when intermediate
smart objects act as IP-forwarders. Following the recommendations in [
?
], we have performed the
simulations in Cooja with a randomly generated seed to automate running the simulations and have
used the default values for Radio Duty Cycling (RDC) in Cooja: the contikimac_driver RDC driver
with a channel check value of 8 Hz.
Sensors 2017,17, 2646 15 of ??
Table 3. Cooja test-bed specifications.
CPU Intel(R) Core(TM) i5-2400 CPU @ 3.10 GHz
RAM 4 GiB DIMM DDR3 Synchronous 1333 MHz
O.S. Ubuntu Server 12.04.5 LTS-32 bits
Kernel 3.13.0-32-generic
1 to n Hops
Cooja Network Simul ati on
Bord er
Route r Controller
AAA
Serve r
TESTBED
6LoWPAN over IEEE 802.15.4
EAP Lower Layer RADIUS(EAP)
Z1
Smart
Object
Figure 5. Cooja test-bed.
We show the different performance measurements gathered with the Cooja simulator to
compare LO-CoAP-EAP performance with CoAP-EAP and PANATIKI. The evaluation is done in
different scenarios: (1) with different numbers of hops ranging from 1–4 and (2) with different lossy
environments with loss ratios of 0.0 0.1 and 0.2. These data have been gathered after performing
around 100 authentications per scenario.
5.2.1. Network Authentication Time in Cooja
Figure
??
shows the network access authentication time. Generally, we can see that PANATIKI
sets an upper bound to the authentication time, while LO-CoAP-EAP with handshake sets the lower
bound. All CoAP-based solutions show a statistically significant different with respect to PANATIKI.
As we can see in Table
??
, the improvement is up to
68% comparing any CoAP-based solution
with PANATIKI. As expected, this difference is partially due to the reduction in message length of the
CoAP-based solutions and the reduction of the number of messages in LO-CoAP-EAP. Since PANA
has longer messages, PANATIKI takes longer to complete an authentication than any CoAP-based
solutions. Among the CoAP based solutions, LO-CoAP-EAP improvement ranges from
26–
53%
with respect to CoAP-EAP, also due to the reduction in size and number of exchanges in LO-CoAP-EAP.
Table 4.
Comparing the improvement of network authentication Time among PANATIKI, CoAP-EAP,
LO-CoAP-EAP without handshake and LO-CoAP-EAP with handshake.
Protocol CoAP-EAP LO-CoAP-EAP
with Handshake
LO-CoAP-EAP
without Handshake
PANATIKI 4.5–39.9% 33.6–62.8% 38.8–67.7%
CoAP-EAP - 26.5–41.3% 35.5–52.8%
LO-CoAP-EAP
with handshake - - 0–28.2%
With CoAP-EAP, there was little difference with PANATIKI in the most favorable conditions
(fewer hops and loss ratio), whereas with LO-CoAP-EAP with or without handshake, the improvement
is noticeable, even with more favorable conditions.
Sensors 2017,17, 2646 16 of ??
0
10
20
30
40
1 2 3 4
Number of Hops
Network Authentication Time
Protocol
CoAP−EAP
LO−CoAP−EAP
LO−CoAP−EAP with handshake
PANATIKI
(a)
0
10
20
30
40
1 2 3 4
Number of Hops
Network Authentication Time
Protocol
CoAP−EAP
LO−CoAP−EAP
LO−CoAP−EAP with handshake
PANATIKI
(b)
0
10
20
30
40
1 2 3 4
Number of Hops
Network Authentication Time
Protocol
CoAP−EAP
LO−CoAP−EAP
LO−CoAP−EAP with handshake
PANATIKI
(c)
Figure 6.
Median network authentication time in Cooja for LO-CoAP-EAP, CoAP-EAP and PANATIKI.
(
a
) Median network authentication time with 0.0 loss ratio; (
b
) median network authentication time
with 0.1 loss ratio; (c) median network authentication time with 0.2 loss ratio.
Sensors 2017,17, 2646 17 of ??
Figure
??
shows the success ratio with different loss ratios: 0.0
??
a, 0.1
??
b and 0.2
??
c. When
the packet loss ratio increases, the possibility of completing a bootstrapping procedure decreases. In
this sense, we can see that PANATIKI sets a lower bound to the success ratio, while LO-CoAP-EAP sets
an upper bound. CoAP-based solutions demonstrate a better performance in every packet loss ratio in
comparison with PANATIKI. As mentioned before, the length and quantity of messages to exchange
play an important role, not only in the time it takes to complete an authentication, but also to complete
the authentication successfully.
As can be seen, PANATIKI is not able to finish authentications with three hops. Table
??
shows
that the improvement ranges from
5–100% comparing any CoAP-based solution with PANTIKI.
CoAP-based solutions are able to complete the authentication generally with a greater success ratio,
since the reduction in the number of messages and their shorter message length has an impact on the
overall number of exchanges and a reduction in fragmentation. Among the CoAP-based solutions,
in the more favorable cases, the improvement is negligible and increases as the conditions are more
severe. LO-CoAP-EAP improvement with respect to CoAP-EAP goes up to 43%. This difference can
also be attributed to the reduction in message length and the number of messages.
0.00
0.25
0.50
0.75
1.00
1234
Number of Hops
Success Ratio
Protocol
CoAP−EAP
LO−CoAP−EAP
LO−CoAP−EAP with handshake
PANATIKI
(a)
0.00
0.25
0.50
0.75
1.00
1234
Number of Hops
Success Ratio
Protocol
CoAP−EAP
LO−CoAP−EAP
LO−CoAP−EAP with handshake
PANATIKI
(b)
Figure 7. Cont.
Sensors 2017,17, 2646 18 of ??
0.00
0.25
0.50
0.75
1.00
1234
Number of Hops
Success Ratio
Protocol
CoAP−EAP
LO−CoAP−EAP
LO−CoAP−EAP with handshake
PANATIKI
(c)
Figure 7.
Success ratio in Cooja for LO-CoAP-EAP, CoAP-EAP and PANATIKI. (
a
) Success ratio with
0.0 loss ratio; (b) success ratio with 0.1 loss ratio; (c) success ratio with 0.2 loss ratio.
Table 5. Success ratio comparison.
Protocol CoAP-EAP LO-CoAP-EAP
with Handshake
LO-CoAP-EAP
without Handshake
PANATIKI 4.9–100% 5.4–100% 5.4–100%
CoAP-EAP - 0–25.2% 0.5–57.5%
LO-CoAP-EAP
with handshake - - 0–43.2%
When evaluating the time to complete the network access authentication, it is worth noting that
this process is done prior to the smart object being able to send or receive data traffic. This means that
how much time it takes to finish the authentication is not so important. However, it is important to
finish it even in harsh conditions. This is even more relevant in LP-WAN where the time to transmit
and receive messages is considerable. For example, with a 0.2 loss ratio and four hops, LO-CoAP-EAP
takes
18 s to complete the network access authentication. This time is reasonable because: (1) we are
assuming a very constrained link; (2) this process is only done once, before the smart object can do
anything else; (3) it will only be done again in case the device looses its state or it resets.
5.2.2. Energy Consumption in Cooja
To perform the evaluation of the energy consumption, we used the Powertrace [
?
] tool that
comes with the Cooja simulator. We use it to estimate the median energy consumed by each network
authentication (mJ/network authentication) in LO-CoAP-EAP and CoAP-EAP (PANATIKI is also
set as the reference). There are different measurements: CPU consumption when the smart object
is fully operative (not in low power or sleeping mode); the consumption when transmitting (TX),
receiving (RX) and the total energy consumption. For the sake of simplicity, we show the total energy
consumption per authentication as a representative measurement, since it gives a general view of
the requirements of each solution in terms of energy consumption. The total energy consumption is
measured for three different loss ratio scenarios; 0.0, 0.1 and 0.2, respectively; and for different hops
ranging from 1–4 hops between the smart object and border router.
Figure
??
shows the energy consumption with different loss ratios: 0 Figure
??
a, 0.1 Figure
??
b
and 0.2 Figure
??
b. The energy consumption, is greatly affected by the energy dedicated to sending
Sensors 2017,17, 2646 19 of ??
and receiving messages. As the number and length of the messages increase, the energy consumption
increases, as well. This is aggravated by the fragmentation of the messages, the retransmissions, etc.
The improvement, as shown in Table
??
, ranges from 7–63% comparing any CoAP-based solution
with PANTIKI. The greater the length of the messages, more bytes are sent over the network. This is
worsened when fragmentation occurs in a message, hindering the completion of an authentication.
In the particular case of PANATIKI, it also has a more aggressive retransmission policy than CoAP,
which results in sending more traffic over a constrained network. Among the CoAP-based solutions,
LO-CoAP-EAP improvement ranges from 23–50% with respect to CoAP-EAP. The reduction in size and
number of messages is also a factor in the reduction of the energy consumption of LO-CoAP-EAP over
COAP-EAP. This is also noticeable comparing LO-CoAP-EAP with and without handshake. The added
exchange of the handshake also influences significantly the energy consumption, as can be appreciated
in Figure ??.
0
10
20
30
40
50
1234
Number of Hops
mJ/Network Access Authentication
Protocol
CoAP−EAP
LO−CoAP−EAP
LO−CoAP−EAP with handshake
PANATIKI
(a)
0
10
20
30
40
50
1234
Number of Hops
mJ/Network Access Authentication
Protocol
CoAP−EAP
LO−CoAP−EAP
LO−CoAP−EAP with handshake
PANATIKI
(b)
Figure 8. Cont.
Sensors 2017,17, 2646 20 of ??
0
10
20
30
40
50
1234
Number of Hops
mJ/Network Access Authentication
Protocol
CoAP−EAP
LO−CoAP−EAP
LO−CoAP−EAP with handshake
PANATIKI
(c)
Figure 8.
Total energy consumption in Cooja for LO-CoAP-EAP, CoAP-EAP and PANATIKI. (
a
) Total
energy consumption with a 0.0 loss ratio; (
b
) total energy consumption with a 0.1 loss ratio; (
c
) total
energy consumption with a 0.2 loss ratio.
Table 6. Energy consumption comparison.
Protocol CoAP-EAP LO-CoAP-EAP
with Handshake
LO-CoAP-EAP
without Handshake
PANATIKI 6.7–32.8% 35.3–54.7% 49.3–63.3%
CoAP-EAP - 23.2–32.6% 40.4–49.5%
LO-CoAP-EAP
with handshake - - 15.8–27.7%
5.2.3. Memory Footprint in Cooja
Table
??
shows the memory footprint of each implementation. First, we show for reference the
memory footprint of an empty program, representing the memory use of the O.S. After that, we show
the memory footprint of each solution that includes the empty program measurements. The two
columns represent how much memory is used in both ROM and RAM. The empty program uses
20 kB of ROM and
3.4 kB of RAM; PANATIKI uses
47 kB of ROM and
6 kB of RAM; CoAP-EAP
47.5 kB of ROM and
5.5 kB of RAM; and LO-CoAP-EAP
48 kB of ROM and
5.5 kB of RAM.
These values include the O.S., the necessary network modules, the EAP state machine, the EAP method
and the EAP lower layer. For a fair comparison, we use the same EAP state machine and EAP method.
Comparing the results, we can say that all EAP lower layers have a similar memory footprint.
In PANATIKI, the O.S. employs
43.5% of ROM and 57% of RAM. For both CoAP-EAP and
LO-CoAP-EAP, the S.O represents
43% ROM and
62% of RAM. The differences between CoAP-EAP
and CoAP-EAP are in terms of code, to handle the case where it is handshake or not. Even though
the CoAP-based solutions use more memory dedicated to the code, this is not an issue, since by
using CoAP, we are sharing a common library present in most IoT devices (a CoAP implementation).
PANATIKI would have to add the CoAP library to the existing code to support CoAP services, and
this is one of the advantages of using a CoAP-based EAP lower layer for IoT (see more details in [
?
]).
Sensors 2017,17, 2646 21 of ??
Table 7. Memory footprint of the implementations in Cooja.
Implementation ROM (bytes) RAM (bytes)
Empty Program 20,505 3410
PANATIKI 47,151 6006
CoAP-EAP 47,601 5484
LO-CoAP-EAP 48,019 5484
5.3. LoRaFabian Network Test-Bed
For the test with LP-WAN, we use a real LP-WAN deployment: the LoRa network in Rennes,
France. A snapshot of part of the city of Rennes where the deployment is located is shown in Figure
??
.
The deployment consists of three antennas (LoRa base stations) from Kerlink covering a fair portion
of the city. Two of the antennas are installed on the structures maintained by TDF (Telediffusion de
France), the company that provides telecommunication services in France, and the closest antenna is
the one that is installed in the IMTatlantique’s campus itself (previously known as Telecom Bretagne).
The location of the measurement where the end device was used is more than 100 m away from the
antenna in IMT atlantique.
Figure 9. LoRaFabian network in Rennes, France.
The setup is shown in Figure
??
. It has a star topology, which means the end nodes can reach
the gateway in a single hop. The smart object instantiated in the end-device (from froggy factory
(www.froggyfactory.com)) runs on Contiki and has an embedded LoRa radio coupled with an Arduino
board to derive its power.
According to the architecture presented in Section
??
, the controller will be in the gateway. Because
it was not possible to avoid disrupting the production deployment, the controller was located in an
external entity. For the gateway to communicate with the external authenticator, the CoAP messages
were sent over HTTP to the controller that has a Python hook enabled that serves as a proxy to transfer
the CoAP packets from HTTP to UDP. Although separated, in this test-bed, the controller and gateway
would be co-located in the same entity in a production deployment. Finally, the AAA server runs
FreeRADIUS Version 2.0.2 (freeradius.org).
Sensors 2017,17, 2646 22 of ??
Smart Object
End-device
Controller
Gateway
Figure 10. Architecture of the LoRaFabian network.
In this test-bed, the nearest antenna is located <100 m from the end-device. These data have been
gathered after performing 15 authentications per parameter to obtain network access authentication
time and energy consumption.
The LoraServer sends a beacon every 30 s, which is broadcast to the network using the antenna.
The end device, upon receiving the beacon from the antenna, shall register itself to the network by
sending a response to the beacon with its hardware address after a random delay in order to avoid
collision with other devices. The network shall send a message to the devices by specifying the device’s
hardware address as the destination address in the 802.15.4 frame. The end device is continuously
listening on the channel frequency except for the period of transmission.
5.3.1. Network Authentication Time in LoRa
A measurement that characterizes each solution of network authentication is the time it takes
to complete. The graphs showing the median network authentication times can be seen in Figure
??
.
As may be appreciated in those figures, we can say that there is a statistically significant improvement
in the network authentication time in LO-CoAP-EAP over CoAP-EAP.
Figure 11. Network authentication time.
To measure the network authentication, we have to consider that the Round Trip Time (RTT)
constitutes the major portion of the network access authentication time in this test-bed. It is the time
that is measured between two successive message received by the end device from the LoRa antenna,
and it includes the travel time over the air, the message processing time in the Python hook, the
authenticator and the RADIUS server. The total network authentication time is the interval that is
Sensors 2017,17, 2646 23 of ??
measured between the time when the end device sends the first message to trigger the authentication
mechanism (Phase I) and the time when the ACK for the last request containing the AUTH option
is sent for the key confirmation (Phase V). Figure
??
shows the time taken by the end devices at
each phase and the total time of the authentication process for one instance of each of the solutions
(CoAP-EAP, LO-CoAP-EAP and LO-CoAP-EAP with handshake).
We can see that at Phase I, all solutions spend a similar time to complete this task. After that,
CoAP-EAP and LO-CoAP-EAP with handshake show a similar time as a consequence of the handshake
exchange. On the contrary, LO-CoAP-EAP saves this time. Regarding Phase III where the EAP identity
is exchanged, only CoAP-EAP engages in that exchange. Phase IV, common to all solutions, where the
EAP method is exchanged, presents similar values in the time spent during the exchange. Finally, for
the final exchange, where the AUTH option is present, we can also see similar values for all solutions.
As we can see, LO-CoAP-EAP has fewer exchanges and less bytes traveling in the network, and
for these reasons, it takes lesser time (15 s) to complete the network access authentication as compared
to the CoAP-EAP (25 s). LO-CoAP-EAP with handshake takes 20 s, an improvement of 5 s over
CoAP-EAP, which is still a considerable improvement. Thus, we have achieved a reduction of
20% in
the authentication time for LO-CoAP-EAP performing the handshake and a reduction of
40% in time
for LO-CoAP-EAP. The question if this is a reasonable time has to take into account that this process
is done one time, before the device can access the network. This is a necessary step to secure the
communications and manage the network. Furthermore, the limited bandwidth in LP-WAN (LoRa in
this case) bound longer transmission times, so we think that these times are not unusual.
As a side note and apart from the differences with the Cooja simulations, we can see that we have
similar values in LP-WAN to the worst cases in Cooja (with a 0.2 loss ratio and 3–4 hops). This gives
us an approximation of the harsh conditions LP-WAN networks are dealing with to transmit data
over the network.
5.3.2. Energy Consumption in LoRaFabian
As there is no Powertrace application support for the LoRa platform, we have used a digital
multimeter to measure the current drawn by the shield containing the smart object, as in Figure
??
during the network authentication process.
The smart object is in idle mode until it receives the first radio packet and continues to work in
radio mode throughout the network authentication process.
Figure 12. Energy measurement setup.
The parameters used to measure the energy consumption are the following: the nominal power
for operation is 5 V; the current consumption in each mode is: 15 mA when idle and 19.6 mA in
transmission (TX) and reception (RX). With these values, we measure the energy consumption of
each solution.
Sensors 2017,17, 2646 24 of ??
Table 8. Energy measurement in LoRaFabian.
CoAP-EAP network authentication 2453.92 mJ/authentication
LO-CoAP-EAP (with handshake) network authentication 1964.9 mJ/authentication
LO-CoAP-EAP (without handshake) network authentication 1473.92 mJ/authentication
Table
??
shows the energy consumption of the test done in LoRa. Comparing the energy
consumption of the solutions and taking as reference CoAP-EAP, we appreciate a reduction of
20%
in energy consumption for LO-CoAP-EAP performing the handshake and a reduction up to
40%
in energy consumption for LO-CoAP-EAP. This clearly shows a considerable improvement over the
previous solution.
5.3.3. Memory Footprint in LoRaFabian
Table
??
shows the memory use for the implementations. For these experiments, we use the
STM32F103RB mote, which as 128 kBytes of ROM and 20 kBytes of SRAM. For reference, we show the
memory footprint of an empty program and after that the values of CoAP-EAP and LP-CoAP-EAP.
The empty program uses
8 kB of ROM and
2.6 kB of RAM. Both CoAP-based solutions have
a similar memory footprint; CoAP-EAP
40.7 kB of ROM and
8.7 kB of RAM; LO-CoAP-EAP
41.1 kB of ROM and 8.7 kB of RAM.
The empty program is an estimate of the memory footprint of the O.S. CoAP-EAP and
LO-CoAP-EAP include, additionally, the necessary network modules, the EAP state machine,
the EAP method and the EAP lower layer. The EAP state machine and EAP method are the same in all
instances. Comparing the results, we can say that for both CoAP-EAP and LO-CoAP-EAP, the O.S.
represents
20% ROM and
33% of RAM. The differences between CoAP-EAP and LO-CoAP-EAP
are in terms of code, due to the changes to manage the situation with handshake.
Table 9. Memory footprint of the implementations in LoRa.
Implementation ROM (bytes) RAM (bytes)
Empty Program 7908 2596
CoAP-EAP 40,744 8668
LO-CoAP-EAP 41,144 8668
6. Conclusions and Future Work
In this work, we have highlighted that the inclusion of new radio technologies into the IoT
landscape, known as LP-WAN, has posed an interesting challenge, when network access authentication
is taken into account. LP-WANs pose even more restricted links than those known in LR-WPAN.
In particular, besides the existing constraints in LR-WPAN, in terms of resources such as memory,
CPU and energy consumption, the bandwidth is severely restricted, as well.
We have presented LO-CoAP-EAP, a complete redesign of a previous solution for network access
authentication (CoAP-EAP), to cope with the restrictions imposed by LP-WAN. LO-CoAP-EAP has
been designed to further reduce the number and the length of messages to deal with the very limited
LP-WAN bandwidth. LO-CoAP-EAP still uses CoAP, EAP and AAA to provide scalability, flexibility
and wireless independence, however reducing the number and length of the messages. Moreover,
once the network access authentication is finished successfully, it is possible to provide key material to
different types of LP-WAN security associations to protect the access to the network.
To assess the performance of LO-CoAP-EAP and to show how it overcomes previous work,
we have performed simulations with the Cooja network simulator to confirm that LO-CoAP-EAP
improves CoAP-EAP and PANA in the context of LR-WPAN. Not only that, we have run
LO-CoAP-EAP and CoAP-EAP over a real LoRa network, the LoRaFabian network, as a representative
Sensors 2017,17, 2646 25 of ??
example of LP-WAN, to experimentally prove that the LO-CoAP-EAP provides an improvement in
both LP-WAN and LR-WPAN networks. It is worth noting that further improvements are expected
in the performance once CoAP is adapted for LP-WAN networks, as is currently happening in the
context of the IETF [
?
]. Our design will not change, but CoAP will provide an even more reduced
EAP lower layer, which will further improve the results obtained.
Future work has been planned to cover the case where the smart object has no direct
communication with the controller before the authentication. For example, the smart object may
not have any routable IP address to reach the controller until it is authenticated and authorized to
join the network, for instance as happens in ZigBee networks. In this case, the assistance of a node
belonging to the network that intermediates between the smart object and the controller is required
to perform the network access authentication. Thus, a CoAP-based intermediary may need to be
designed and implemented to perform an authentication in these cases.
Acknowledgments:
We want to thank Antonio F. Gomez-Skarmeta for supporting this work. This paper has also
been made possible by the Spanish National Project CICYT EDISON(TIN2014-52099-R) granted by the Ministry of
Economy and Competitiveness of Spain (including ERDF support).
Author Contributions:
University of Murcia provided the initial idea in which this work is based (CoAP-EAP),
the new design collaborating partially proposing the optimizations and the experiments done with the Cooja
simulator. Acklio provided their expertise in LP-WAN, proposing partially the optimizations to the original
design to adapt to the constrains of LP-WAN, as well as the measurements done with LoRa.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this manuscript:
6LoWPAN IPv6 over Low power Wireless Personal Area Networks
AAA Authentication, Authorization and Accounting
AES Advanced Encryption Standard
CoAP Constrained Application Protocol
DTLS Datagram Transport Layer Security Version
EAP Extensible Authentication Protocol
EAP KMF EAP Key Management Framework
IEEE Institute of Electrical and Electronics Engineers
LP-WAN Low-Power Wide-Area Network
LR-WPAN Low-Rate Wireless Personal Area Network
LoRa Long Range
MSK Master Session Key
NAS Network Access Server
OTA Over The Air
PANA Protocol for Carrying Authentication for Network Access
PSK Pre-Shared Key
RADIUS Remote Authentication Dial In User Service
RPL Routing Protocol for Low-Power and Lossy Networks
SAP Security Association Protocol
TSK Transient Session Key
URI Uniform Resource Identifier
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